The background to the invention is as follows. Copper clad laminates are at present manufactured by taking copper foil produced generally in accordance with the teaching of U.S. Pat. No. 3,674,656, laying it on top of one or more sheets of partially cured resin impregnated base material, and placing the two materials between press plates on a laminating press. Under heat and pressure the partially cured resin adheres to the copper foil so that, when removed from the press, the two materials are firmly bonded together.
Such copper foil as is used in this process is available in unsupported form in a thickness range of 9 .mu.m upwards to in excess of 105 .mu.m. Since such foil is frequently in excess of 1 meter wide, handling sheets of it can be difficult and, particularly in thicknesses between 9 and 20 .mu.m, a great deal of scrap is generated in the laying up process. In order to preserve the surface quality of the laminate great care has to be taken to exclude all dust particles from between the surface of the copper foil and the press plates, which are used to separate the laminates in the press during manufacture.
In order to facilitate the handling of thin copper foils it has been proposed to manufacture such materials by continuously depositing such copper onto a carrier foil of aluminium of chromium-plated copper and processes for so doing are disclosed in U.S. Pat. No. 4,113,576 and U.K. Patent Specifications Nos. 1 460 849, 1 458 260, and 1 458 259. In practice foils produced by these techniques are costly and unreliable and have found little favour in the industry.
U.S. Pat. No. 3,984,598 describes a process in which a stainless steel press plate known as a caul plate is coated with a silane as a release agent and is then electroplated with copper. The exposed surface of the copper is then oxidised and treated with a silane as a bonding agent. The copper-clad caul plate is then laminated to resin-impregnated base material in a laminating press. After the laminate is removed from the press, the caul plate is removed from the copper-clad dielectric board which has been produced. The copper coating is found to have a variable thickness of about 5 to 12 .mu.m.
It is notoriously difficult to uniformly electroplate a surface provided with an organic parting layer such as silane. As a result, the deposit will suffer from porosity. During laminating, the resin will therefore seep through the pores, both causing adherence to the caul plate and contaminating the surface of the laminate.
What is desired is a method by which thin copper layers of 3 microns and upwards can be successfully and economically laminated to dielectric base materials with great reliability.